Google leads me here. @user35915's answer helped me a lot, adding details here. Hope this could help others.

Setup these two commands

command 2
>enable b 3
>c
command 3
>disable b 3
>c

Which means, when 2 is hit, enable 3, then continue. And when 3 is hit, disable 3, then continue.

The disable b 3 in latter command ensures 3 is hit at most once whenever it's enabled.

Append continue to commands saves me from typing c manually. If some detailed observation is needed, I would add commands before c, or even remove c (to stop program there). Like this,

command 3
>disable b 3
>bt
>c